<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>JavaScript异步编程</title>
</head>
<body>
    <h1>函数回调</h1>
    <p class="msg"></p>\
    <script>
        document.addEventListener("DOMContentLoaded",function(){
            let msg=document.querySelector(".msg");
            msg.innerHTML+=doOperation();
            msg.innerHTML="<br>";

            asyncDoOperation(msg);
        });
    </script>
</body>
</html>